CDMA, at least in the general sense, is a form of cellular telecommunications. Cellular telephones are portable radios that enable users, often called subscribers, to communicate with each other through a communications network. This wireless-communication network is sometimes called a public-land mobile network (PLMN), and the PLMN is typically connected to other communications networks so that subscribers may communicate through them and with their subscribers as well. The most familiar of these other networks is the familiar public-switched telephone network (PSTN), sometimes referred to as the plain old telephone system (POTS). The Internet is another well-known network that may be in communication with a PLMN, although it is generally used for data rather than voice traffic. The PLMN may also be connected to smaller networks, such as intranets, local-area networks (LANs), or virtual private networks (VPNs). The PLMN, in general, connects to these other networks through devices called gateways, which help translate the communication from a format understood by one network to a format understood by another. In addition, gateways also enable communications between different PLMNs and possibly different portions of a single PLMN.
Although cellular telephones are radio transmitters, various cellular technologies enable a great many of them to be used within relative proximity to each other. For one thing, while high-powered radio transmitters are capable of transmitting a signal that can be picked up by a receiver miles away, cellular phones communicate within a smaller range, typically one to ten miles. And rather than calling a central network antenna, they communicate with a nearby base station antenna, one associated with the area—or cell—in which they are currently located. The base station, in turn, communicates with the rest of the PLMN to establish a connection for the call. Note that in such a “wireless” network, the “wireless” communication takes place between the subscriber's mobile phone and the base station. This link is sometimes referred to as the “air interface”. The rest of the network is fixed in location and generally interconnected by wire, cable, or fiber (although radio frequency, microwave frequency, infra-red, or some other technology may also be used).
At this point, it should also be noted that as the terms “cellular (or cell) phone” and “mobile phone” are sometimes used interchangeably, and will be treated as equivalent herein. Both, however, are a sub-group of a larger family of devices that also includes, for example, certain computers and personal digital assistants (PDAs) that are also capable of wireless radio communication in a cellular network. This family of devices will for convenience be referred to as “mobile stations” (regardless of whether a particular device is actually moved about—or even capable of being moved).
FIG. 1 is a simplified block diagram illustrating the configuration of a typical PLMN 100. As mentioned previously, the entire geographic area covered by such a network (which is not shown in FIG. 1) is divided into a number of cells, such as cells 10 through 15 delineated by broken lines in FIG. 1. Although only six cells are shown, there are typically a great many. In the illustrated embodiment, each cell has associated with it a base transceiver station (BTS) for example BTS 20 for transmitting and receiving messages to and from mobile stations (MS) in cell 10, here MS 31, MS 32, and MS 33, via radio frequency (RF) links 35, 36, and 37, respectively. Mobile stations MS 31 through MS 33 are usually (though not necessarily) mobile, and free to move in and out of cell 10. Radio links 35-37 are therefore established only where necessary for communication. When the need for a particular radio link no longer exists, the associated radio channels are freed for use in other communications. (Certain channels, however, are dedicated for beacon transmissions and are therefore in continuous use.) BTS 21 through BTS 25, located in cell 11 through cell 15, respectively, are similarly equipped to establish radio contact with mobile stations in the cells they cover.
BTS 20, BTS 21, and BTS 22 operate under the direction of a base station controller (BSC) 26, which also manages communication with the remainder of PLMN 100. Similarly, BTS 23, BTS 24, and BTS 25 are controlled by BSC 27. In the PLMN 100 of FIG. 1, BSC 26 and 27 are directly connected and may therefore both communicate and switch calls directly with each other. Not all BSCs in PLMN 100 are so connected, however, and must therefore communicate through a central switch. To this end, BSC 20 is in communication with mobile switching center MSC 29. MSC 29 is operable to route communication traffic throughout PLMN 100 by sending it to other BSCs with which it is in communication, or to another MSC (not shown) of PLMN 100. Where appropriate, MSC 29 may also have the capability to route traffic to other networks, such as a packet data network 50. Packet data network 50 may be the Internet, an intranet, a local area network (LAN), or any of numerous other communication networks that transfer data via a packet-switching protocol. Data passing from one network to another will typically though not necessarily pass through some type of gateway 49, which not only provides a connection, but converts the data from one format to another, as appropriate. Note that packet data network 50 is typically connected to the MSC 29, as shown here, for low data rate applications. Where higher data rates are needed, such as in 3G CDMA networks, the packet data network 50 is connected directly to the BSCs (26, 27) which in such networks are capable of processing the packet data.
A cellular wireless system such as the one illustrated in FIG. 1 has several advantages over a central antenna system. As the cells are much smaller than the large geographic area covered by a central antenna, transmitters do not need as much power. This is particularly important where the transmitter is housed in a small device such as a cell phone. In addition, the use of low-power transmitters means that although the number of them operating in any one cell is still limited, the cells are small enough that a great many may operate in an area the size of a major city. The mobile stations do not transmit with enough power to interfere with others operating in different cells (not adjoining the one they are in). In some systems, this enables frequency reuse, that is the same communication frequencies can be used in non-adjacent cells at the same time without interference. In other systems, codes used for privacy or signal processing may be reused in a similar manner.
In addition to the cellular architecture itself, certain multiple access schemes may also be employed to increase the number of mobile stations that may operate at the same time in a given area. In frequency-division multiple access (FDMA), the available transmission bandwidth is divided into a number of channels, each for use by a different caller (or for a different non-traffic use). A disadvantage of FDMA, however, is that each frequency channel used for traffic is captured for the duration of each call and cannot be used for others. Time-division multiple access (TDMA) improves upon the FDMA scheme by dividing each frequency channel into time slots. Any given call is assigned one or more of these time slots on which to send information. More then one voice caller may therefore use each frequency channel. Although the channel is not continuously dedicated to them, the resulting discontinuity is usually imperceptible to the user. For data transmissions, of course, the discontinuity is not normally a factor.
Code-division multiple access (CDMA) operates somewhat differently. Rather than divide the available transmission bandwidth into individual channels, individual transmissions are spread over a frequency band and encoded (as explained more fully below). By encoding each transmission in a different way, each receiver (i.e. mobile station) decodes only information intended for it and ignores other transmissions. The number of mobile stations that can operate in a given area is therefore limited by the number of encoding sequences available, rather than the number of frequency bands.
Returning to FIG. 1, when a mobile station, for example MS 33, leaves cell 10 and enters cell 12, its communication link to the network is transferred from BTS 20 to BTS 22. If MS 33 is inactive, its relocation means only that a radio link will be established with BTS 22 when necessary to originate or terminate a call. If MS 33 is actively engaged in an on-going communication, however, or in the process of call set-up as it moves from one cell to the other, PLMN 100 will attempt to maintain this communication through a process called “handoff”.
Using a predetermined algorithm, MS 33 will determine (or be notified) that handoff is appropriate and will then switch from one BTS to another. Handoffs may be “soft” or “hard”. A hard handoff means that the radio link 38 to BTS 20 is broken before a new link to BTS 22 is established. Preferably, the discontinuity in service is barely perceptible to the subscriber. (It may be highly disruptive to data transmissions, however.) In a soft handoff, active MS 33 will establish radio link 39 with BTS 22 while it is still located in cell 10 (and may establish radio links with other BTSs in other cells as well). MS 33, BTS 20, and BTS 22 cooperate to continually evaluate the relative signal strength of radio links 37 and 39 to determine, according to a predetermined algorithm, when handoff is appropriate. Because radio link 39 is established before radio link 37 is broken, this type of transfer is preferable to the hard alternative because it lessens the interruption of service to the subscriber and the risk of dropping the call entirely. As MS 33 moves from cell 10 to cell 12, this change in location (or, more properly, change in serving BTS) is preferably reflected in the visitor location register (VLR) 28, a database connected with (or incorporated as a part of) MSC 29. By tracking the serving BSS for mobile stations, of course, PLMN 100 can more efficiently establish a connection to a target mobile station. Because they use code division, as opposed to frequency division, CDMA networks typically provide for soft handoffs.
Inactive mobile stations may also, of course, relocate from one cell to another, or even from one network-covered area to another. In this case, the MS location information in a VLR such as VLR 28 may be updated when the mobile station registers. Registration is simply the process of sending out a signal by the mobile station when it is powered-up, and periodically thereafter. The registration signal is picked up by a nearby BTS (and often by more than one), which relays the location information to, for example, VLR 28 through MSC 29. Periodically, VLR 28 will also notify the home location register (HLR) 45, a central database of PLMN 100 that tracks not only the location of mobile stations that subscribe to the PLMN 100, but also subscription information such as the services subscribed to MS capabilities, etc.
When a call directed to a particular mobile station is placed, the location information in HLR 45 and the various VLRs is checked so that the call can be appropriately routed. A page or other incoming-call notification is broadcast by the BTS serving the cell where the mobile station's location was last recorded and, if the mobile station responds, a radio link terminating the call is established. If the mobile station does not respond to this page, the PLMN may send out paging messages in other cells in an attempt to locate the target mobile station. If, after a period of time, all such pages are unsuccessful, the PLMN returns an appropriate message so that the originating caller can be notified that the target mobile station is unavailable. If the service is available, the disappointed call originator may be given the opportunity to leave a voice or numeric message that is recorded on a centrally accessible database (not shown) from which it can be transmitted to the intended call recipient at a later time.
The PLMN 100 described above has been discussed in the context of a call originating or terminating at a single mobile station. Such a call may, of course, involve a second mobile station, even one in the same cell as the mobile station with which it is communicating. Although such a call may be routed through less of the PLMN, with respect to each party, it is conducted as any other call. In other words, the BCS will direct the establishment of a radio link with each mobile station in the same fashion as it would if the call originated (or terminated) outside of the cell. Even if the call routing is done entirely within the BCS, the radio transmissions to each party will follow the standard method. While this is appropriate for a call involving two mobile stations, in other situations an alternative procedure may be preferred.
One such situation involves multicasting. “Multicasting” refers to a manner of sending a given message to a selected group of recipients. (For definitional purposes, such a group could include none or only one recipient, although typically such transmissions are intended for a larger group.) Multicasting may best be understood in contrast to “broadcasting”, in which a message is transmitted to all recipients equipped with proper receiving devices, and in contrast to a connection established for private communication between a single sender and a single receiver. Significantly, multicasting does not include the process of simply sending individual but identical messages to a number of users. While this alternative would produce, from the recipient's perspective, the same result as multicasting, it would not be as efficient in terms of system resources. Rather, multicasting provides for the conservation of system resources by transmitting a message so that it is received (or, at least, is available for reception) by members of a defined group and by no others. In other words, multicasting involves not only sending a single message to a plurality of users, but a particular way of doing so as well. Multicasting may be used to send a single message, a sequence of discrete messages, or an extended transmission such as a streaming-video multimedia presentation.
Multicasting is a concept familiar in the Internet-protocol (IP) environment. IP is a packet-switched technology, with information segmented into separately-addressed packets that are sent from a source node to a recipient node. If a source node is sending the same content to multiple recipients, several options are available. One option, of course, is for the human operator to simply execute the sending function a number of times, or program a computer to do so. This is an inefficient use of both the operator's resources and those of the network, however, a deficiency that IP multicasting was developed to correct.
In an IP network, this multicast function is accomplished by assigning a multicast address to the members of a multicast group. Each of these terminals already has (and maintains) a unique IP unicast address, and receives messages directed there. Multicast addresses, which are taken from a set of addresses reserved for the purpose, are assigned when the group is created. A group is created when one member indicates its desire to initiate a group or to subscribe to an existing group (whose multicast address it has learned by other means). In Internet protocol version 4 (IPv4), for example, this would be done using an Internet Group Management Protocol (IGMP) message. In IPv6, a Multicast Listener Discovery (MLD) message would be used.
Significantly, when a multicast group is established and a multicast address is assigned to the group, the multicast message is sent to each member (assuming they are listening for messages) without being copied multiple times at the originating router. The network simply fans out the message (packet stream) in the direction of the group members until the message arrives at each.
While multicasting is a familiar process in other types of networks, especially in packet-switched data networks such as Ethernet local area networks (LANs), no corresponding procedure is yet available in cellular networks operating according to a CDMA standard. A scheme for efficiently facilitating multicast in the CDMA environment is needed and would permit CDMA networks to realize the efficiencies associated with such transmissions. The system and method of the present invention provide just such a solution.